Charging circuit

ABSTRACT

Charging a storage cell requires the electromotive force exerted at a photogenerating cell in addition to the voltage equal to or higher than the forward on voltage developed at an backflow preventing diode. Therefore, the charging is inefficient. Moreover, the area of the backflow preventing diode must be large in consideration for a current supply from the photogenerating cell at a high intensity of illumination.  
     A charging circuit, constructed using a differential amplifier, which has a power supply therefor separated from another power supply, is used as a direction-of-current detecting circuit that detects the direction of current from a voltage difference between two different power supplies. Consequently, a switch is logically turned on or off depending on whether charging or non-charging is under way. Thus, on voltage to be developed during charging is lowered. Moreover, the size or area of a transistor that acts as a logical circuit is made smaller than that of the backflow preventing diode. Furthermore, the energy of a storage cell included in the charging circuit is hardly consumed in any states.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a charging circuit capable ofefficiently performing charging by detecting the direction of currentfrom a voltage difference between two different power supplies.

[0003] 2. Description of the Related Art

[0004] A charging circuit 6 having two different power supplies, astorage cell 2 and a photogenerating cell 30, and a backflow preventingdiode 40, similar to the one shown in FIG. 6, has been known in thepast.

[0005] The charging circuit 6 uses the storage cell 2 thereof to drive adrive circuit 5. When a voltage produced at the photogenerating cell 30is higher than the one at the storage cell 2, the photogenerating cell30 can charge the storage cell 2. The photogenerating cell 30 has apositive electrode having a reference potential 1 of the chargingcircuit 6, and a negative electrode having a power supply potential ofthe charging circuit 6.

[0006] The photogenerating cell 30 adopts the structure of a pn junctionhaving a p-type semiconductor and an n-type semiconductor joinedtogether. Specifically, four pn junctions are connected in series withone another in order to exert an electromotive force of approximately2.8 v.

[0007] The backflow preventing diode 40 is connected between the storagecell 2 and photogenerating cell 30 so that the direction of current thatflows from the photogenerating cell 30 to the storage cell 2 willcorrespond to the forward direction of the backflow preventing diode.

[0008] Moreover, the drive circuit 5 driven by the charging circuit 6 isconnected between the positive electrode (reference potential 1) and thenegative electrode (supply potential).

[0009] Next, the actions of the charging circuit 6 shown in FIG. 6 willbe described below.

[0010] To begin with, a description will be made of a case where thevoltage at the storage cell 2 is lower than the one at thephotogenerating cell 30.

[0011] A reverse current produced by the photogenerating cell 30 servesas the charging current for the storage cell 2. The direction of thecurrent corresponds to the forward direction of the backflow preventingdiode 40. The flow of the current is therefore not prevented but thestorage cell 2 is charged. Incidentally, the forward voltage developedat the backflow preventing diode 40, through which the current flows inthe forward direction, is approximately 0.4 V. Therefore, unless thevoltage difference between the photogenerating cell 30 and storage cell2 is equal to or larger than 0.4V, charging cannot be achieved inpractice.

[0012] Next, a description will be made of a case where the voltage atthe storage cell 2 is equal to or higher than the one at thephotogenerating cell 30.

[0013] When the voltage at the storage cell 2 is equal to the one at thephotogenerating cell 30, the voltages are balanced. No reverse currenttherefore flows from the photogenerating cell 30. When the voltage atthe storage cell 2 is higher than the one at the photogenerating cell30, current attempts to flow from the storage cell 2 to thephotogenerating cell 30. However, as the direction of the currentcorresponds to the reverse direction of the backflow preventing diode40, the flow of the current to the storage cell 2 is blocked.

[0014] Moreover, the backflow preventing diode 40 is realized with ametal-oxide semiconductor field-effect transistor (MOSFET) having astructure called a diode-connected structure in which the gate and drainthereof are shorted. Incidentally, only a voltage equal to the thresholdvoltage of the transistor is applied as the gate voltage.

[0015] However, if the voltage difference between the photogeneratingcell 30 and storage cell 2 is so large that the charging currentincreases, the supply of current to the backflow preventing diode 40must be increased. Therefore, the backflow preventing diode 40 isstructured so that the ratio of a gate width to a gate length relevantto the diode-connected MOSFET will be large.

[0016] When the backflow preventing diode 40 is adopted, if the voltagedifference between the photogenerating cell 30 and storage cell 2 issmall (approximately 0.4 V or less) or if electromotive force is limitedbecause no light falls on the photogenerating cell 30 (low intensity ofillumination), charging is not achieved efficiently. Moreover, the areaof the backflow preventing diode 40 must be increased in order to ensurea sufficient supply of current. This leads to an increase in the area ofa system LSI in which the charging circuit 6 is incorporated.

[0017] As a means for solving the above problems, a method according towhich an operational amplifier is used to sense a voltage differencebetween two different power supplies for the purpose of logicallyswitching between charging and non-charging has been disclosed in U.S.Pat. No. 4,291,266.

[0018] However, according to the method, a storage cell to be charged isused to drive the operational amplifier. The operational amplifier istherefore driven during non-charging. Consequently, the energy of thestorage cell is consumed. This poses a problem especially when super-lowpower is used for driving.

SUMMARY OF THE INVENTION

[0019] Accordingly, an object of the present invention is to provide acharging circuit and an electronic timepiece which include adirection-of-current detecting circuit that detects the direction ofcurrent from a voltage difference between two different power supplies.

[0020] Another object of the present invention is to provide a chargingcircuit and an electronic timepiece in which the energy of a storagecell is not consumed even during non-charging.

[0021] Still another object of the present invention is to provide acharging circuit and an electronic timepiece that can be designedcompactly and incorporated in an LSI.

[0022] In order to accomplish the above objects, a charging circuit inaccordance with the present invention comprises a storage cell, agenerating cell, a switch element, a reference current producing circuitfor producing reference current using the generating cell as a powersupply, and a comparative control circuit which uses the referencecurrent to compare the voltage at the storage cell with the voltage atthe generating cell, and which turns on the switch element so as tocharge the storage cell, using the generating cell, when the voltage atthe generating cell is higher than the voltage at the storage cell, andwhich turns off the switch element so as to prevent release of energyfrom the storage cell to the generating cell when the voltage at thegenerating cell is lower than the voltage at the storage cell.

[0023] In order to accomplish the aforesaid objects, an electronictimepiece in accordance with the present invention comprises, a drivecircuit for driving the movement of an electronic timepiece, a storagecell for use in supplying power to the drive circuit, a generating cell,a switch element, a reference current producing circuit that produces areference current using the generating cell as a power supply, and acomparative control circuit which uses the reference current to comparethe voltage at the storage cell with the voltage at the generating cell,and which turns on the switch element so as to charge the storage cellusing the generating cell when the voltage at the generating cell ishigher than the voltage at the storage cell, and which turns off theswitch element so as to prevent release of energy from the storage cellto the generating cell when the voltage at the generating cell is lowerthan the voltage at the storage cell.

[0024] Preferably, the generating cell is a photogenerating cell, athermogenerating cell, or a mechanical generating cell.

[0025] Preferably, the comparative control circuit has a common load,and the reference current producing circuit causes a reference currentto flow into the common load.

[0026] More preferably, the comparative control circuit includes a firsttransistor, a second transistor, a first load, a second load, and acommon load. The other terminal of the common load is connected to oneterminal of each of the generating cell and storage cell. The firstterminal of the first transistor is connected to one terminal of thecommon load, the second terminal thereof is connected to one terminal ofthe first load, and the third terminal thereof is connected to the otherterminal of the storage cell. The first terminal of the secondtransistor is connected to one terminal of the common load, the secondterminal thereof is connected to one terminal of the second load, andthe third terminal thereof is connected to the other terminal of thegenerating cell. The other terminal of the first load is connected tothe other terminal of the generating cell. The other terminal of thesecond load is connected to the other terminal of the storage cell. Thecomparative control circuit is connected to the switch element throughthe second terminal of the second transistor serving as the outputterminal thereof.

[0027] More preferably, the first transistor, second transistor, firstload, and second load are formed with MOSFETs. The conductivity type ofthe first and second transistors is different from that of the first andsecond loads.

[0028] More preferably, the ratio of a gate width to a gate lengthrelevant to the second transistor is larger than that relevant to thefirst transistor.

[0029] More preferably, one terminal of the generating cell and oneterminal of the storage cell are connected to each other. The otherterminal of the generating cell and one terminal of the switch elementare connected to each other. The other terminal of the storage cell andthe other terminal of the switch element are connected to each other.

[0030] More preferably, the switch element is formed with a MOSFET.

[0031] According to the present invention, a differential amplifierhaving a power supply therefor separated from the storage cell is usedas a direction-of-current detecting circuit that detects the directionof current from a voltage difference between two different powersupplies. The switch element is logically controlled depending onwhether charging or non-charging is under way. Consequently, a voltageto be developed during charging is lowered.

[0032] Moreover, the energy of the storage cell is not consumed duringnon-charging. A circuit for charging the storage cell is actuated usingthe photogenerating cell during charging. In both the charging andnon-charging states, therefore, the power consumption of the storagecell can be minimized.

[0033] Furthermore, according to the present invention, the switchelement formed with a MOSFET is substituted for the backflow preventingdiode formed with a diode-connected MOSFET. As long as a permissiblecurrent is the same, the size of the switch element can be reduced byapproximately {fraction (1/2)} power of that of the backflow preventingdiode. Consequently, if the charging circuit is fabricated in the formof an LSI, the charging circuit will be very compact.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a circuit diagram showing an example of a chargingcircuit in accordance with the present invention;

[0035]FIG. 2 shows waveforms indicating the operational states of thecharging circuit in accordance with the present invention;

[0036]FIG. 3 shows waveforms indicating the operational states of thecharging circuit in accordance with the present invention;

[0037]FIG. 4 shows waveforms indicating the operational states of thecharging circuit in accordance with the present invention;

[0038]FIG. 5 is a circuit diagram showing a case where the chargingcircuit shown in FIG. 1 is adapted to an electronic timepiece; and

[0039]FIG. 6 is a circuit diagram showing a charging circuit inaccordance with a related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040]FIG. 1 is a circuit diagram showing the configuration of acharging circuit 3 in accordance with a preferred embodiment of thepresent invention.

[0041] The charging circuit 3 shown in FIG. 1 comprises a storage cell2, a constant-voltage source 10, a differential amplifier 20, a switchelement 29, and a photogenerating cell 30. Referring to FIG. 1, thestorage cell 2 has a positive electrode having a reference potential 1of the charging circuit 3, and a negative electrode having a powersupply potential of the charging circuit 3.

[0042] In FIG. 1, a lithium-ion secondary batterry is used as thestorage cell 2. Moreover, the constant-voltage source 10, differentialamplifier 20, and switch element 29 are formed with MOSFETs.

[0043] The photogenerating cell 30 adopts the structure of a pn junctionhaving a p-type semiconductor and an n-type semiconductor joinedtogether. When light is irradiated to the pn junction, recombination ofcarriers takes place and electric energy is produced. At this time, theproduced reverse current serves as charging current. In general,electromotive force exerted at a one-stage pn junction is approximately0.7 V. A plurality of stages of pn junctions is connected in series withone another in order to increase the electromotive force. In FIG. 1, thephotogenerating cell 30 has four stages of pn junctions connected inseries with one another so that an electromotive force of approximately2.8 v will be exerted.

[0044] The constant-voltage source 10 comprises a reference resistor 11,a diode-connected third p-type MOSFET 12, a third n-type MOSFET 15, afourth p-type MOSFET 13, and a diode-connected fourth n-type MOSFET 14.Moreover, the reference resistor 11 is connected between the referencepotential 1 and the source of the fourth p-type MOSFET 13. The drain ofthe fourth p-type MOSFET 13 is connected to the drain of the fourthn-type MOSFET 14. The source of the fourth n-type MOSFET is connected tothe negative electrode (power supply potential) of the photogeneratingcell 30. Furthermore, the third p-type MOSFET 12 is connected betweenthe reference potential 1 and the drain of the third n-type MOSFET 15.The gate of the third p-type MOSFET 12 is connected to the gate of thefourth p-type MOSFET 13. The gate of the third n-type MOSFET 15 isconnected to the gate of the fourth n-type MOSFET 4. The source of thethird n-type MOSFET 15 is connected to the negative electrode (powersupply potential) of the photogenerating cell 30.

[0045] The constant-voltage source 10 is of a so-called bandgapreference type. The MOSFETS included in the constant-voltage source 10operate at voltages near their threshold voltage. The ratio of a gatewidth to a gate length relevant to each of the MOSFETs and theresistance offered by the reference resistor 11 determine the voltage tobe developed at an output node 12 a. The constant-voltage source 10 isdesigned so that when it operates perfectly, the output voltage willremain constant. This kind of constant-voltage source 10 has theproperty of absorbing a change in ambient temperature or a fluctuationin the threshold voltage of a transistor.

[0046] The constant-voltage source 10 operates using the photogeneratingcell 30 as a power supply and acts as a means for causing constantcurrent (reference current) to flow into a common load 21 included inthe differential amplifier 20 that detects the direction of current. Aslong as the output voltage of the constant-voltage source 10 remainsconstant, the voltage to be applied to the gate of the common load 21formed with a p-type MOSFET is kept constant. Consequently, constantcurrent flows into the common load 21.

[0047] The differential amplifier 20 comprises a first p-type MOSFET 27,a second p-type MOSFET 25, a first load 28, a second load 26, and thecommon load 21.

[0048] The common load 21 is formed with a p-type MOSFET, and works tocause constant current to flow so that the voltage at the output node 12a included in the constant-voltage source 10 will be applied as the gatevoltage of the p-type MOSFET. Moreover, the common load 21 is connectedto the third p-type MOSFET-12 included in the constant-voltage source10, whereby a current mirror is realized. The current flowing into thecommon load 21 is determined with the current flowing into the thirdp-type MOSFET 12 and the ratio of a gate width to a gate length relevantto each of the common load 21 and third p-type MOSFET 12. If both thecommon load 21 and third p-type MOSFET 12 share the same ratio of a gatewidth to a gate length, the same current flows into the third p-typeMOSFET and common load. In the charging circuit 3 shown in FIG. 1, thecommon load 21 and third p-type MOSFET 12 share the same ratio of a gatewidth to a gate length. However, the present invention is not limited tothis circuitry.

[0049] Incidentally, the common load 21 may be formed with a resistorand not with the p-type MOSFET. However, in this case, the currentvaries linearly relative to the voltage applied to the resistor. It istherefore preferred to include a means for externally producing aconstant current.

[0050] In the differential amplifier 20, the first p-type MOSFET 27 andsecond p-type MOSFET 25, and the first load 28 and second load 26, areopposed to each other. The sources of the first p-type MOSFET 27 andsecond p-type MOSFET 25 are connected to the drain of the common load21. The source of the common load 21 is connected to the referencepotential 1. The first load 28 and second load 26 are formed with n-typeMOSFETS. In other words, the conductivity type of the first transistor27 and second transistor 25 is different from that of the first load 28and second load 26. The drain of the first load 28 is connected to thedrain of the first p-type MOSFET 27, and the drain of the second load 26is connected to the drain of the second p-type MOSFET 25. The secondload 26 is formed with a diode-connected transistor having the drain andgate thereof shorted. The source of the first load 28 is connected tothe negative electrode (power supply potential) of the storage cell 2,and the source of the second load 26 is connected to the negativeelectrode (power supply potential) of the photogenerating cell 30.Furthermore, the gate of the first load 28 and the gate of the secondload 26 are connected to each other.

[0051] The output node 27 a included in the differential amplifier 20 isconnected to the gate of the switch element 29 formed with an n-typeMOSFET. When the current flowing into the first p-type MOSFET 27 isdifferent from the current flowing into the second p-type MOSFET 25, thecommon load 21 attempts to cause a constant current to flow. Thedifferential amplifier 20 therefore operates so that both the firstp-type MOSFET 27 and second p-type MOSPET 25 will cause the same currentto flow. Consequently, the voltage difference between thephotogenerating cell 30 and storage cell 2 is provided, as an outputvoltage, through the output node 27 a.

[0052] As mentioned above, the differential amplifier 20 detects thevoltage difference between the photogenerating cell 30 and storage cell2, controls the gate voltage of the switch element 29, and controls thedrain current of the switch element 29.

[0053] Next, the actions of the charging circuit 3 will be describedbelow.

[0054] To begin with, a description will be made of a case where thevoltage at the photogenerating cell 30 is higher than the one at thestorage cell 2.

[0055] In this case, the storage cell 2 is charged using thephotogenerating cell 30 (charging state). Moreover, the current flowingto the constant-voltage source 10 that uses the photogenerating cell 30as a power supply becomes constant. Consequently, a constant voltage isdeveloped at the output node 12 a.

[0056] The common load 21 included in the differential amplifier 20 andthe third p-type MOSFET 12 included in the constant-voltage source 10are connected to each other, thus constituting a current mirror.Therefore, if both the common load 21 and third p-type MOSFET 12 sharethe same ratio of a gate width to a gate length, the same current flowsinto them. Moreover, the differential amplifier 20 operates to retainthe current flowing into the common load 21 at a constant value all thetime. In the charging state, the gate of the second p-type MOSFET 25included in the differential amplifier 20 is brought to an on stateaccording to the voltage at the photogenerating cell 30. At this time,the gate voltage of the diode-connected second load 26 shifts towardsthe power supply potential. As the voltage at the photogenerating cell30 gets larger than that at the storage cell 2, the gate voltage of thesecond load 26 shifts more towards the power supply potential.Furthermore, the gate voltage of the first load 28 opposed to the secondload 26 shifts towards the power supply potential at the same time. Thisbrings the first load 28 to an off state. Along with this action, theoutput voltage at the output node 27 a included in the differentialamplifier 20 is determined. As the voltage at the photogenerating cell30 becomes larger than that at the storage cell 2, the output voltage atthe output node 27 a shifts more towards the reference potential 1. Thegate voltage of the switch element 29 is controlled based on the outputvoltage at the output node 27 a. Therefore, as the voltage at thephotogenerating cell 30 becomes larger than that at the storage cell 2,the gate voltage of the switch element 29 shifts more towards thereference potential 1. This leads to an increase in the on current thatflows through the switch element 29. This state is the charging state inwhich the storage cell 2 is charged with the reverse current produced atthe photogenerating cell 30.

[0057] Next, a description will be made of a case where the differencebetween the voltage at the photogenerating cell 30 and the voltage atthe storage device 2 is small (including a case where the voltages atthe photogenerating cell 30 and storage cell 2 are equal to each other).

[0058] Within a transition region which the voltage difference betweenthe photogenerating cell 30 and the storage device 2 changes little, thegate voltage of the second p-type MOSFET 25 included in the differentialamplifier 20 is almost the same as the voltage at the photogeneratingcell 30. When the voltage at the photogenerating cell 30 graduallychanges, from a state in which it is much higher than the voltage at thestorage cell 2 to a state in which it is close to the voltage at thestorage cell 2, the gate voltage of the second load 26 shifts from thevalue attained in the charging state towards the value of the referencepotential 1 (however, does not exactly reach the reference potential 1).Furthermore, when the voltage at the photogenerating cell 30 becomescloser to the voltage at the storage cell 2, charging and non-chargingare switched. At this time, a feed-through current flows between thereference potential 1 included in the differential amplifier 20 and thepower supply potential. Thereafter, even if the voltage at the storagecell 2 becomes higher than that at the photogenerating cell 30, as longas the voltage difference between the storage cell 2 and thephotogenerating cell 30 remains small and the gate voltage of the firstload 28 does not reach the reference potential 1, the switch element 29is not fully turned off. However, as mentioned above, even if thevoltage difference between the storage cell 2 and the photogeneratingcell 30 remains small, since it is the photogenerating cell 30 thatdrives the differential amplifier 20, the consumed energy of the storagecell 2 is nearly zero.

[0059] Moreover, the timing of switching charging and non-charging canbe changed arbitrarily. For example, the ratio of a gate width to a gatelength relevant to the second p-type MOSFET 25 is made larger than theone relevant to the first p-type MOSFET 27 so that the timing ofbringing the second p-type MOSFET 25 included in the differentialamplifier 20 to the off state will come earlier than the timing ofbringing the first p-type MOSFET 27 to the off state. In this case, whenthe voltage making the voltage at the photogenerating cell 30 equal tothe voltage at the storage cell 2 is lower by an offset voltage, thesecond p-type MOSFET 25 is brought to the off state. When the secondp-type MOSFET 25 is brought to the off state, charging is switched tonon-charging. Incidentally, the offset voltage is determined with theratio of the ratio of a gate width to a gate length relevant to thefirst p-type MOSFET 27 to the ratio of a gate width to a gate lengthrelevant to the second p-type MOSFET 25 then the offset voltage isdetermined this way, the feed-through current flowing through thedifferential amplifier 20 at the time of switching between charging andnon-charging can be reduced.

[0060] Next, a description will be made of a case where the voltage atthe photogenerating cell 30 is lower than that at the storage cell 2 anda case where the voltage at the photogenerating cell 30 decreases to beequal to or lower than the threshold voltage of the transistors includedin the constant-voltage source 10.

[0061] After the voltage at the photogenerating cell 30 changes thestates from the transition region as mentioned above, it becomes lowerthan that at the storage cell 2. Consequently, the gate voltage of thesecond load 26 shifts towards the reference potential 1, and the switchelement 29 is turned off.

[0062] As mentioned above, a constant voltage is normally provided asthe output voltage at the output node 12 a in the constant-voltagesource 10. However, if the voltage at the photogenerating cell 30becomes equal to or lower than the threshold voltage of the transistorsincluded in the constant-voltage source 10, the output voltage providedat the output node 12 in the constant-voltage source 10 rapidlydecreases and approaches 0 V. In other words, the constant-voltagesource 10 does not function any longer. For example, when the thresholdvoltage of the transistors included in the constant-voltage source 10 is0.5 V, and if the voltage at the photogenerating cell 30 becomes equalto or lower than 00.5 V, the constant-voltage source 10 cannot providethe constant voltage (that is, cannot supply reference current to thecommon load 21). When the output voltage at the output node 12 a in theconstant-voltage source 10 decreases to approach 0 V, the gate voltageof the common load 21 included in the differential amplifier 20decreases and the differential amplifier 20 halts completely. In thisstate, the storage cell 20 alone is used to drive a system circuit suchas a drive circuit for a electronic timepiece. Moreover, in this state,the switch element 29 is turned off. Consequently, only the leakagecurrent caused by the transistor realizing the switch element 29 flows.Namely, reverse current does not flow from the storage cell 2 to thephotogenerating cell 30. As mentioned above, the constant-voltage source10 operates using the photogenerating cell 30 as a power supply.Consequently, when the voltage at the photogenerating cell 30 decreases,the constant-voltage source 10 cannot operate any longer. The currentflowing into the common load 21 diminishes, and the differentialamplifier 20 itself does not operate. When the charging circuit shown inFIG. 1 enters the non-charging state, the constant-voltage source 10 anddifferential amplifier 20 do not operate. The energy of the storage cell2 is then hardly consumed. Furthermore, even in the charging state, asthe constant-voltage source 10 uses the photogenerating cell 30 as apower supply, the power in the storage cell 2 will not be consumed inorder to operate the constant-voltage source 10. Namely, the chargingcircuit 3 is advantageous in the point that the energy of the storagecell 2 is hardly consumed during either charging or non-charging.

[0063] In the conventional charging circuit 6 shown in FIG. 6, chargingand non-charging are passively switched by the backflow preventing diode40. In contrast, in the charging circuit 3 in accordance with thepresent invention, the differential amplifier is used to monitorsubstantially two power supplies (photogenerating cell and storagecell). Thus, the direction of current is actively detected.

[0064] According to the related art shown in FIG. 6, the backflowpreventing diode 40 is formed with a diode-connected MOSFET having thegate and drain thereof shorted. Only voltage equal to the thresholdvoltage of the MOSFET is applied as the gate voltage of the backflowpreventing diode 40. Consequently, the size of the backflow preventingdiode 40 must be increased in order to ensure a required supply ofcurrent.

[0065] In contrast, according to the present invention, the switchelement formed with a MOSFET is substituted for the backflow preventingdiode formed with the diode-connected MOSFET. As long as the permissiblecurrent value is the same, the size of the switch element can be reducedby approximately {fraction (1/2)} power of the size of the backflowpreventing diode. This is apparent from the fact that the drain currentis plotted as a curve of a square function relative to the gate voltage.

[0066] Referring to FIG. 2 to FIG. 4, the actions of the chargingcircuit 3 in accordance with the present invention will be describedbelow.

[0067]FIG. 2 is a graph showing a change in the voltage at the outputnode 27 a in the differential amplifier 20 in relation to the voltage atthe photogenerating cell 30 which occurs with the voltage at the storagecell 2 included in the charging circuit 3 shown in FIG. 1 held constant.

[0068] Referring to FIG. 2, the abscissa indicates the voltage at thephotogenerating cell 30, and the ordinate indicates the voltage at theoutput node 27 a of the differential amplifier 20. Moreover, the curves101, 102, 103, and 104 are concerned with cases where the voltage at thestorage cell 2 is −0.5 V, −1.0 V, −1.5 V, or −2.0 V.

[0069] As seen from FIG. 2, when the absolute value of the voltage atthe photogenerating cell 30 is larger than the absolute value of thevoltage at the storage cell 2, the output voltage of the differentialamplifier 20 is closer to the reference potential 1 (0 V). In contrast,when the absolute value of the voltage at the photogenerating cell 30 isequal to or smaller than the absolute value of the voltage at thestorage cell 2, the output voltage of the differential amplifier 20becomes lower than the reference potential 1.

[0070] The difference between the output voltage of the deferentialamplifier 20 when the voltage at the photogenerating cell 30 is higherthan the one at the storage cell 2 and the output voltage of thedeferential amplifier 20 when the voltage at the photogenerating cell 30is equal to or lower than the one at the storage cell 2 is approximately0.7 v.

[0071]FIG. 3 is a graph indicating a change in the drain current of theswitch element 29 in relation to the voltage at the photogenerating cell30 which occurs with the voltage at the storage cell 2, included in thecharging circuit 3 shown in FIG. 1, held constant.

[0072] Referring to FIG. 3, the abscissa indicates the voltage at thephotogenerating cell 30, and the ordinate indicates the drain current ofthe switch element 29. Moreover, the curves are concerned with thevarious values of the voltage at the storage cell 2. Namely, the curves101 a, 102 a, 103 a, and 104 a are concerned with the values of thevoltage at the storage cell 2, that is, −0.5 V, −1.0 V, −1.5 V, and −2.0V respectively.

[0073] The gate voltage of the switch element 29 is controlled with theoutput voltage of the differential amplifier 20. Consequently, thechange in the drain current of the switch element 29 corresponds to thechange in the voltage at the output node 27 a of the differentialamplifier 20 occurring in relation to the change in the voltage at thephotogenerating cell 30 as shown in FIG. 2.

[0074] As shown in FIG. 3, when the absolute value of the voltage at thephotogenerating cell 30 is larger than the absolute value of the voltageat the storage cell 2, current flows into the switch element 29.However, when the difference between the voltage at the photogeneratingcell 30 and that at the storage cell 2 gets smaller, the currentdecreases gradually when the absolute value of the voltage at thephotogenerating cell 30 becomes equal to or smaller than that at thestorage cell 2, no current flows.

[0075] This signifies that when the absolute value of the voltage at thephotogenerating cell 30 is larger than the absolute value of the voltageat the storage cell 2, the switch element 29 is turned on with theoutput voltage of the differential amplifier 20. In contrast, when theabsolute value of the voltage at the photogenerating cell 30 is equal toor smaller than that at the storage cell 2, the switch element 29 isturned off.

[0076]FIG. 4 indicates the output voltage of the differential amplifier20 and the current flowing into the first p-type MOSFET 27 included inthe differential amplifier 20 in relation to the voltage at thephotogenerating cell 30.

[0077] A curve 103 indicates the relationship between the voltage at thephotogenerating cell 30 and the output voltage of the differentialamplifier 20 which is established with the voltage at the storage cell 2held at −1.5 V as indicated in FIG. 2.

[0078] The curve 103 is concerned with a case where the ratio of a gatewidth to a gate length relevant to the second p-type MOSFET 25 includedin the differential amplifier 20 is the same as the one relevant to thefirst p-type MOSFET 27 moreover, a dashed line 106 is concerned with acase where the ratio of a gate width to a gate length relevant to thesecond p-type MOSFET 25 included in the differential amplifier 20 islarger than the one relevant to the first p-type MOSFET 27. In thiscase, an input offset voltage is applied. A dot-dash line 107 isconcerned with a case where the ratio of a gate width to a gate lengthrelevant to the second p-type MOSFET 25 included in the differentialamplifier 20 is smaller than the one relevant to the first p-type MOSFET27 in this case, no input offset voltage is applied.

[0079] The changes in the current flowing into the first p-type MOSFET27 included in the differential amplifier 20 which occur in the cases103, 106, and 107 respectively are indicated with lines 103 b, 106 b,and 107 b respectively.

[0080] As shown in FIG. 4, the output voltage of the differentialamplifier 20 shifts towards the reference potential 1 (0 V) in relationto the absolute value of the voltage at the photogenerating cell thatdecrease in the order of the cases 106, 103, and 107. Moreover, thecurrent flowing into the switch element 29 included in the differentialamplifier 20 increases in the order of the changes in the currentflowing into the first p-type MOSFET 27 indicated with the lines 103 b,106 b, and 107 b respectively.

[0081] As the ratio of a gate width to a gate length relevant to thefirst p-type MOSFET 27 is smaller than that relevant to the secondp-type MOSPET 25 included in the differential amplifier 20, the inputoffset voltage becomes higher. Moreover, the current flowing into thefirst p-type MOSFET diminishes. Accordingly, the output voltage of thedifferential amplifier 20 shifts from near the reference potential 1 (0v in FIG. 4) towards a lower voltage.

[0082] This signifies that when a voltage difference between thephotogenerating cell 30 and storage cell 2 is detected, the voltagedifference is detected as a voltage containing the offset voltage. Forexample, normally, when the voltage at the photogenerating cell 30 isequal to or lower than that at the storage cell 2, the switch element 29is turned off. However, if the input offset voltage is applied, itbecomes critical whether a voltage produced by adding up the voltage atthe photogenerating cell 30 and the input offset voltage is equal to orlower than the voltage at the storage cell 2. Consequently, the voltagecausing the switch element 29 to switch off shifts from near thereference potential 1 (0 v in FIG. 4) towards a lower voltage by theinput offset voltage.

[0083] As mentioned above, the second p-type MOSFET 25 and first p-typeMOSFET 27 included in the differential amplifier 20 are used todetermine the input offset voltage and to thus determine the timing ofdriving the switch element 29. Consequently, the input offset voltage(for example, 0.4 V) whose application determines the timing ofswitching charging and non-charging which is fixed in a case where thebackflow preventing diode is employed can be set to several tens ofmillivolts. According to the present embodiment, the ratio of a gatewidth to a gate length relevant to the second p-type MOSFET 25 is madelarger than that relevant to the first p-type MOSFET 27. Thus, the inputoffset voltage whose application determines the timing of switchingcharging and non-charging is set to 40 mV.

[0084] In the charging circuit 6 in accordance with a related art shownin FIG. 6, the backflow preventing diode 40 is used to passively switchcharging and non-charging. In contrast, as mentioned above, the chargingcircuit 3 in accordance with the present invention actively detects thedirection of current.

[0085] Moreover, in the charging circuit 6 of the related art shown inFIG. 6, the backflow preventing diode 40 is formed with adiode-connected MOS transistor having the gate and drain thereofshorted. Consequently, only voltage equivalent to the threshold voltageof the transistor is applied as the gate voltage of the backflowpreventing diode 40. The forward on voltage developed at the backflowpreventing diode 40 is as high as approximately 0.4 V. Moreover, thesize of the backflow preventing diode 40 must be increased in order toensure a sufficient supply of current.

[0086] According to the present invention, the switch element formedwith a MOSFET is substituted for the backflow preventing diode formedwith a diode-connected MOSFET. As long as a permissible current value isthe same, the size of the switch element can be reduced by approximately{fraction (1/2)} power of the size of the backflow preventing diode.This is apparent from the fact that the drain current is plotted as acurve of a square function relative to the gate voltage.

[0087] Furthermore, the power supply for the differential amplifier 20that detects a voltage difference between two different power suppliesis separated from another power supply. One of the two power supplies isthe storage cell 2 or a power supply connected on the storage cell 2side and the other thereof is the photogenerating cell 30 or a powersupply connected on the photogenerating cell 30 side. Moreover, thepower supply for the constant voltage source 10 that controls thecurrent flowing into the common load 21 included in the differentialamplifier 20 is connected on the photogenerating cell 30 side.Consequently, the consumption of the energy of the storage cell 2 can beminimized in all states including the charging state and non-chargingstate.

[0088] Furthermore, as the differential amplifier connected to the twodifferent power supplies is included in the charging circuit, the switchelement is logically turned on or off depending on whether charging ornon-charging is under way. Consequently, the on voltage developed duringcharging can be decreased to several tens of millivolts.

[0089]FIG. 5 shows a case where the charging circuit 3 shown in FIG. 1is adapted to an electronic timepiece 60. In FIG. 5, the same referencenumerals are assigned to components identical to those shown in FIG. 1.

[0090] Referring to FIG. 5, a first switch 51 is connected in parallelto the photogenerating cell 30. The first switch 51 acts as a switch forpreventing overcharging of the storage cell 2. Consequently, when thevoltage at the storage cell 2 becomes equal to or higher than a ratedvoltage, the first switch 51 is turned on. The photogenerating cell 30is shorted in order to lower the inter-node voltage.

[0091] A capacitor 54 is connected on a current path extending from thephotogenerating cell 30 via a second switch 52. The capacitor 54 is usedto rapidly drive a drive circuit 56 in case the voltage at the storagecell 2 has decreased to such an extent that the drive circuit 56 cannotbe driven using the storage cell 2.

[0092] The second switch 52 is shorted when the drive circuit 56 must berapidly driven in order to drive the movement of a electronic timepiece.Moreover, when the voltage at the storage cell 2 rises to such an extentthat the drive circuit 56 can be driven with the voltage at the storagecell 2 (steady driving), the second switch 52 is opened.

[0093] A third switch 53 is connected between the photogenerating cell30 and storage cell 2. The third switch 3 is used to switch rapiddriving and steady driving for the drive circuit 56.

[0094] A voltage detecting means 55 monitors the voltage at the storagecell 2 all the time, and controls the on or off states of the first,second, and third switches.

[0095] Next, actions performed in the electronic timepiece shown in FIG.5 will be described below.

[0096] To begin with, a description will be made of a state in which thedrive circuit 56 is driven with the storage cell 2 (steady driving). Thevoltage detecting means 55 monitors the voltage at the storage cell 2when the voltage at the storage cell 2 exceeds 1.3 V, the second switch52 is turned off. Thereafter, the third switch 53 is turned on in orderto drive the drive circuit 56 using the storage cell 2. Incidentally,the value of 1.3 V is a mere example and can be modified variouslyaccording to a situation.

[0097] In this state, as mentioned above, the differential amplifier 2included in the charging circuit 3 references the voltages at thephotogenerating cell 30 and storage cell 2 so as to switch the chargingand non-charging states. The voltage at the storage cell 2 must not beallowed to be equal to or higher than the rated voltage for the purposeof preventing deterioration from charging. Therefore, the voltagedetecting means 55 monitors the inter-node voltage of the storage cell2. When the inter-node voltage exceeds 2.6 V, the first switch is turnedon in order to short the photogenerating cell 3.0. This causes thevoltage at the photogenerating cell 30 to drop, whereby the voltage atthe storage cell 2 is prevented from rising to be equal to or higherthan 2.6 V. The value of 2.6 V is a mere example and may be modifiedvariously according to a situation.

[0098] Next, a description will be made of a case where the voltage atthe storage cell 2 has decreased to such an extent that the drivecircuit 57 cannot be driven with the voltage, and the drive circuit 56is halted.

[0099] In this state, the first switch 51 is off, the second switch 52is on, and the third switch 53 is off. If light is irradiated to thephotogenerating cell 30 in this state, the capacitor 54 as well as thestorage cell 2 is charged. Thereafter, the drive circuit 56 is rapidlydriven using the capacitor 54.

[0100] In the charging circuit 3 shown in FIG. 1 and the electronictimepiece 60 shown in FIG. 5, a photogenerating cell 30 is adopted as agenerating cell. Alternatively, a thermogenerating cell or a mechanicalgenerating cell may be substituted for the photogenerating cell 30.Regarding the thermgenerating cell, a thermogenerating cell designed toutilize heat energy generated by a human body and formed using atelluric bismuthate (BiTe) alloy is available. Moreover, the mechanicalgenerating cell includes a compact automatic-wind generator having agenerating rotor that rotates with conveying driving torque. Moreover,when the generating voltage developed by the thermogenerating cell ormechanical generating cell is low, a booster or the like may beemployed.

What is claimed is:
 1. A charging circuit comprising: a storage cell; agenerating cell; a switch element; a reference current producing circuitfor producing reference current using said generating cell as a powersupply; and a comparative control circuit which uses said referencecurrent to compare the voltage at said storage cell with the voltage atsaid generating cell, and which turns on said switch element so as tocharge said storage cell using said generating cell when the voltage atsaid generating cell is higher than the voltage at said storage cell,and which turns off said switch element so as to prevent release ofenergy from said storage cell to said generating cell when the voltageat said generating cell is lower than the voltage at said storage cell.2. The charging circuit according to claim 1, wherein said generatingcel is a photogenerating cell.
 3. The charging circuit according toclaim 1, wherein said generating cell is a thermogenerating cell.
 4. Thecharging circuit according to claim 1, wherein said generating cell is amechanical generating cell.
 5. The charging circuit according to claim1, wherein said comparative control circuit is a differential amplifierhaving a common load, and said reference current producing circuitcauses said reference current to flow into said common load.
 6. Thecharging circuit according to claim 1, wherein: said comparative controlcircuit includes a first transistor, a second transistor, a first load,a second load, and a common load; the other terminal of said common loadis connected to one terminal of each of said generating cell and storagecell; a first terminal of said first transistor is connected to oneterminal of said common load, a second terminal thereof is connected toone terminal of said first load, and a third terminal thereof isconnected to the other terminal of said storage cell; a first terminalof said second transistor is connected to one terminal of said commonload, a second terminal thereof is connected to one terminal of saidsecond load, and a third terminal thereof is connected to the otherterminal of said generating cell; the other terminal of said first loadis connected to the other terminal of said generating cell; the otherterminal of said second load is connected to the other terminal of saidstorage cell; and said comparative control circuit is connected to saidswitch element through said second terminal of said second transistorserving as an output terminal of said comparative control circuit. 7.The charging circuit according to claim 6, wherein said firsttransistor, second transistor, first load, and second load are formedwith MOSFETs, and the conductivity type of said first transistor andsecond transistor is different from that of said first load and secondload.
 8. The charging circuit according to claim 7, wherein the ratio ofa gate width to a gate length relevant to said second transistor islarger than that relevant to said first transistor.
 9. The chargingcircuit according to claim 1, wherein one terminal of said generatingcell and one terminal of said storage cell are connected to each other,the other terminal of said generating cell and one terminal of saidswitch element are connected to each other, and the other terminal ofsaid storage cell and the other terminal of said switch element areconnected to each other.
 10. The charging circuit according to claim 9,wherein said switch element is formed with a MOSFET.
 11. An electronictimepiece comprising: a drive circuit for driving the movement of aclock; a storage cell for supplying power to said drive circuit; agenerating cell; a switch element; a reference current producing circuitfor producing reference current using said generating cell as a powersupply; and a comparative control circuit which uses said referencecurrent to compare the voltage at said storage cell with the voltage atsaid generating cell, and which turns on said switch element so as tocharge said storage cell using said generating cell when the voltage atsaid generating cell is higher than the voltage at said storage cell,and which turns off said switch element so as to prevent release ofenergy from said storage cell to said generating cell when the voltageat said generating cell is lower than the voltage at said storage cell.12. The electronic timepiece according to claim 11, wherein saidgenerating cell is a photogenerating cell.
 13. The electronic timepieceaccording to claim 11, wherein said generating cell is athermogenerating cell.
 14. The electronic timepiece according to claim11, wherein said generating cell is a mechanical generating cell. 15.The electronic timepiece according to claim 11, wherein said comparativecontrol circuit is a differential amplifier having a common load, andsaid reference current producing circuit causes said reference currentto flow into said common load.
 16. The electronic timepiece according toclaim 11, wherein: said comparative control circuit includes a firsttransistor, a second transistor, a first load, a second load, and acommon load; the other terminal of said common load is connected to oneterminal of each of said generating cell and storage cell; a firstterminal of said first transistor is connected to one terminal of saidcommon load, a second terminal thereof is connected to one terminal ofsaid first load, and a third terminal thereof is connected to the otherterminal of said storage cell; a first terminal of said secondtransistor is connected to one terminal of said common load, a secondterminal thereof is connected to one terminal of said second load, and athird terminal thereof is connected to the other terminal of saidgenerating cell; the other terminal of said first load is connected tothe other terminal of said generating cell; the other terminal of saidsecond load is connected to the other terminal of said storage cell; andsaid comparative control circuit is connected to said switch elementthrough said second terminal of said second transistor serving as anoutput terminal of said comparative control circuit.
 17. The electronictimepiece according to claim 16, wherein said first transistor, secondtransistor, first load, and second load are formed with MOSFETs, and theconductivity type of said first transistor and second transistor isdifferent from that of said first load and second load.
 18. Theelectronic timepiece according to claim 17, wherein the ratio of a gatewidth to a gate length relevant to said second transistor is larger thanthat relevant to said first transistor.
 19. The electronic timepieceaccording to claim 11, wherein one terminal of said generating cell andone terminal of said storage cell are connected to each other, the otherterminal of said generating cell and one terminal of said switch elementare connected to each other, and the other terminal of said storage celland the other terminal of said switch element are connected to eachother.
 20. The electronic timepiece according to claim 19, wherein saidswitch element is formed with a MOSFET.